U.S. patent number 4,908,926 [Application Number 07/288,248] was granted by the patent office on 1990-03-20 for method of and apparatus for controlling nut runner.
This patent grant is currently assigned to Honda Giken Kogyo Kabushiki Kaisha. Invention is credited to Toshikazu Asakura, Shoichi Hayashi, Shigeo Kobayashi, Akihiko Takahashi, Akira Takeshima.
United States Patent |
4,908,926 |
Takeshima , et al. |
March 20, 1990 |
Method of and apparatus for controlling nut runner
Abstract
The output shaft of a nut runner for tightening a fastener such
as a bolt is first rotated with a first torque at a first speed.
When the tightening torque of the output shaft reaches a preset
switching torque, the first torque first speed rotation of the
output shaft is interrupted, and thereafter the output shaft is
rotated with a second torque at a second speed. When the tightening
torque then reaches a preset snug torque, the second torque second
speed rotation of the output shaft is interrupted, and thereafter
the output shaft is rotated with a third torque at a third speed.
When the tightening torque reaches a preset final torque, the third
torque third speed rotation of the output shaft is stopped, thus
completing the tightening of the fastener.
Inventors: |
Takeshima; Akira (Sayama,
JP), Asakura; Toshikazu (Sayama, JP),
Takahashi; Akihiko (Sayama, JP), Kobayashi;
Shigeo (Sayama, JP), Hayashi; Shoichi (Sayama,
JP) |
Assignee: |
Honda Giken Kogyo Kabushiki
Kaisha (Tokyo, JP)
|
Family
ID: |
18159585 |
Appl.
No.: |
07/288,248 |
Filed: |
December 22, 1988 |
Foreign Application Priority Data
|
|
|
|
|
Dec 23, 1987 [JP] |
|
|
62-323876 |
|
Current U.S.
Class: |
29/407.02;
173/181; 29/446; 81/469 |
Current CPC
Class: |
B25B
21/008 (20130101); B25B 23/147 (20130101); G05D
17/02 (20130101); Y10T 29/49863 (20150115); Y10T
29/49766 (20150115) |
Current International
Class: |
B25B
21/00 (20060101); B25B 23/14 (20060101); B25B
23/147 (20060101); G05D 17/00 (20060101); G05D
17/02 (20060101); B23Q 017/00 () |
Field of
Search: |
;29/407,428,446,240
;81/469 ;173/1,12 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
2720803 |
October 1955 |
Rice et al. |
3827506 |
August 1974 |
Himmelstein et al. |
3965778 |
June 1976 |
Aspers et al. |
4095325 |
June 1978 |
Hashimoto et al. |
4413396 |
November 1983 |
Wallace et al. |
4791838 |
December 1988 |
Bickford et al. |
|
Foreign Patent Documents
Primary Examiner: Gorski; Joseph M.
Claims
What is claimed is:
1. A method of controlling a nut runner for tightening a fastener,
comprising the steps of:
rotating an output shaft of the nut runner with a predetermined
first torque at a predetermined first speed;
detecting said predetermined first torque of said output shaft;
interrupting the rotating of said output shaft when said
predetermined first torque reaches a preset switching torque
immediately before said fastener is seated on a surface;
thereafter, rotating said output shaft with a second torque, said
second torque being greater than said predetermined first torque,
and at a second speed, said second speed being slower than said
predetermined first speed;
interrupting the rotation of said output shaft when said second
torque reaches a preset snug torque when said fastener is seated on
the surface;
thereafter, rotating said output shaft with a third torque, said
third torque being greater than said predetermined first torque and
said second torque, and at a third speed, said third speed being
slower than said predetermined first speed and said second speed;
and
stopping the rotating of said output shaft when said third torque
reaches a preset final torque when said fastener is completely
tightened.
2. The method according to claim 1, further including the step
of:
removing torque from said output shaft when interrupting the
rotation of said output shaft immediately before said fastener is
seated on the surface and also when interrupting the rotation of
said output shaft when said fastener is seated on the surface.
3. The method according to claim 2, further including the step
of:
reversing the rotation of said output shaft, thereby braking the
output shaft, and thereby removing torque from said output
shaft.
4. The method according to claim 1, wherein said snug torque is
about 20% of said final torque and said switching torque is about
70% of said snug torque.
5. The method according to claim 1, wherein said predetermined
first speed ranges from 600 rpm to 800 rpm, said second speed
ranges from 100 rpm to 200 rpm, and said third speed ranges from 10
rpm to 30 rpm.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of and an apparatus for
controlling a nut runner which tighten a fastener such as a bolt, a
nut, or the like by reducing step-wise the rotational speed of the
output shaft of the nut runner.
2. Description of the Background
Nut runners are employed to tighten fasteners such as bolts, nuts,
or the like. When tightening such a fastener with a nut runner, in
order to reduce the time required to tighten the fastener, the
fastener is first tightened with a low torque and at a high speed
until the tightening torque reaches a prescribed level from the
start of tightening the fastener. Then, when the prescribed torque
level is reached, the fastener is tightened with a higher torque at
a low speed. Such a two-stage tightening process is performed by
rotating the output shaft of the nut runner first at a higher speed
and then at a lower speed.
One known fastener tightening process of the above type is
disclosed in Japanese Patent Publication No. 53-3840, for example.
In the disclosed process, the fastener is tightened at a high speed
by a nut runner driven by a DC motor until a prescribed torque
level is reached.
The current supplied to the DC motor and the torque produced by the
DC motor are proportional to each other. Therefore, the torque of
the DC motor can be detected by detecting the value of the current
supplied thereto. When the torque thus detected of the nut runner
output shaft reaches a certain switching torque level, the power
supply that energizes the DC motor is switched from a
higher-voltage unit to a lower-voltage unit to lower the voltage
applied to the DC motor, thereby rotating the nut runner output
shaft at a lower speed. When the torque of the nut runner output
torque, as detected by a separate torque sensor, reaches a
prescribed tightening torque level, the DC motor is de-energized
completing the tightening process.
In the above tightening process, the rotational speed of the motor
is switched from a higher speed to a lower speed immediately before
the fastener, such as a bolt, is seated on a surface. Since the
tightening torque of the output shaft increases abruptly just
before the fastener is seated, however, the rotational speed of the
motor may not appropriately be switched from the higher speed to
the lower speed on account of an error in the motor current
measured for torque detection, a fluctuation in the rotational
speed of the motor, a delay in the response to the switching
between the motor speeds, an inertial force of a rotating member
such as the output shaft, and other factors. If the motor speed is
switched from the higher speed to the lower speed after the
fastener is seated, then the tightening torque is increased in
excess of the desired torque level due to the response delay in the
switching from the higher to the lower speed. Particularly,
inasmuch as the fastener has been tightened at a high speed
immediately before it is seated, the inertial force of the output
shaft is large, and the fastener tends to be excessively tightened
due to such large inertial force even after a switching signal to
change the motor speed from the higher speed to the lower speed is
issued. Such a switching signal may be applied earlier to change
the motor speed from the higher speed to the lower speed in order
to avoid an excessive tightening of the fastener. Then, the motor
speed may be switched to the lower speed too early because of a
measured motor current, a motor speed fluctuation, or the like,
with the result that the fastener may be tightened at the lower
speed for a certain period of time before it is seated, and the
total tightening time required may be prolonged.
To prevent the above problems, it is necessary to lower the
rotational speed of the nut runner output shaft to reduce the
inertial force thereof when the rotational speed is high.
Typically, the rotational speed of the output shaft is reduced to
about 300 rpm. Therefore, efforts to reduce the total tightening
time are limited.
U.S. Pat. No. 3,965,778 discloses a procedure for reliably
switching the rotational speed of the output shaft of a nut runner
from a higher speed to a lower speed. According to the disclosed
process, when a prescribed tightening torque is detected at a snug
point immediately before a bolt or the like is seated on a surface,
the motor is de-energized to stop the rotation of the output shaft
so as to maintain an output shaft torque corresponding to a
fastener seating torque.
However, since a torque increase from the tightening torque at the
snug point to a final tightening torque is extremely abrupt, a
certain response delay may nevertheless occur and the fastener may
still be excessively tightened when the output shaft is stopped at
the final tightening torque level.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of and
an apparatus for controlling a nut runner by detecting a tightening
torque of the output shaft of the nut runner, switching the
rotational speed of the output shaft between three speed stages,
i.e., higher, or predetermined speed medium, and lower speeds or
first, second, and third speeds, temporarily stopping the output
shaft upon speed switching to eliminate adverse effects of a
response delay and an inertial force of the output shaft, and
increasing the higher rotational speed to shorten a time required
to tighten a fastener.
To achieve the above object, there is provided a method of
controlling a nut runner for tightening a fastener, including the
following steps of: rotating an output shaft of the nut runner with
a lower predetermined torque at a higher, predetermined speed;
detecting a tightening torque of the output shaft; and interrupting
the rotation of the output shaft when the detected tightening
torque reaches a preset switching torque immediately before the
fastener is seated on a surface. Thereafter, the output shaft is
rotated with a medium torque relative to the lower, predetermined
torque and at a medium speed relative to the higher, predetermined
speed followed by; interrupting the rotation of the output shaft
when the detected tightening torque reaches a preset snug torque
when the fastener is seated on the surface. Finally, the output
shaft is rotated with a higher torque relative to the lower,
predetermined torque and at a lower speed relative to the higher,
predetermined speed; and the rotation of the output shaft is
stopped when the detected tightening torque reaches a preset final
torque when the fastener is completely tightened. The lower, medium
and higher torque, identified above and subsequently, may also be
described as a first, second and third torque, respectively.
Similarly, the higher, medium and lower speed, identified above and
subsequently, may be described as a first, second and third speed,
respectively.
The method further includes the step of removing the tightening
torque from the output shaft when interrupting the rotation of the
output shaft immediately before the fastener is seated on the
surface and also when interrupting the rotation of the output shaft
when the fastener is seated on the surface.
The method further includes the step of reversing the rotation of
the output shaft to brake the output shaft to remove the tightening
torque from the output shaft.
The snug torque is about 20% of the final torque and the switching
torque is about 70% of the snug torque.
The higher speed ranges from 600 rpm to 800 rpm, the medium speed
ranges from 100 rpm to 200 rpm, and the lower speed ranges from 10
rpm to 30 rpm.
According to the present invention, there is also provided an
apparatus for controlling a nut runner for tightening a fastener,
including a motor for rotating an output shaft of the nut runner, a
speed reducer including a switching clutch shiftable selectively in
higher and lower speed positions for rotating the output shaft in
respective higher and lower speed ranges and a torque sensor for
detecting a tightening torque of the output shaft. A master
controller includes means for presetting a switching torque, a snug
torque, a final torque, a higher speed, a medium speed, and a lower
speed. A subcontroller includes comparator means for determining
whether the detected tightening torque reaches each of the preset
switching, snug, and final torques, switching means responsive to a
signal from the comparator means for rotating the output shaft with
a lower torque at the higher speed with the switching clutch in the
higher speed position, interrupting the rotation of the output
shaft and shifting the switching clutch into the lower speed
position when the detected tightening torque reaches the preset
switching torque. Thereafter, the output shaft is rotated with a
medium torque at the medium speed, and interrupts the rotation of
the output shaft when the detected tightening torque reaches the
preset snug torque. Finally, the output shaft is rotated with a
higher torque at the lower speed, and stop the rotation of the
output shaft when the detected tightening torque reaches the preset
final torque. Motor driver means responsive to an output signal
from the switching means as provided for controlling the motor and
the output shaft through the speed reducer.
With the method of the present invention, the switching torque, the
snug torque, and the final torque are preset. The output shaft of
the nut runner is first rotated with the lower torque at the higher
speed. The tightening torque of the output shaft is detected and
compared with the switching torque. When the tightening torque
reaches the switching torque, the lower-torque higher-speed
rotation of the output shaft is interrupted, and thereafter the
output shaft is rotated with the medium torque at the medium speed.
When the tightening torque reaches the snug torque, the
medium-torque medium-speed rotation of the output shaft is
interrupted, and thereafter the output shaft is rotated with the
higher torque at the higher speed. When the tightening torque
reaches the final torque, the higher-torque lower-speed rotation of
the output shaft is stopped thus completing the tightening of the
fastener.
Each time the rotational speed of the output shaft is to be
changed, the rotation of the output shaft is interrupted to prevent
inertial forces of the output shaft from adversely affecting the
tightening of the fastener.
Each time the rotational speed of the output shaft is to be
changed, the tightening torque is removed from the output shaft to
relatively reduce an increase in the tightening torque from the
time immediately before the fastener is seated to the time when the
fastener is seated, thereby facilitating the control of the
position in which the output shaft is stalled.
The snug torque is about 20% of the final torque and said switching
torque is about 70% of the snug torque, and the higher speed ranges
from 600 rpm to 800 rpm, the medium speed ranges from 100 rpm to
200 rpm, and the lower speed ranges from 10 rpm to 30 rpm. With
these numerical settings, the time period required for tightening
the fastener is shortened, and the final tightening torque is of a
desired torque level.
With the control method of the present invention, the master
controller is preset to the switching torque, the snug torque, the
final torque, the higher speed, the medium speed, and the lower
speed. The output shaft is rotated with the lower torque at the
higher speed by the motor through the speed reducer with the
switching clutch in the higher speed position. The tightening
torque detected by the torque sensor is compared with the switching
torque by the comparator means. When the tightening torque reaches
the switching torque, the rotation of the output shaft is
interrupted by the switching means through the motor driver means,
and the switching clutch is shifted into the lower speed position.
Thereafter, the output shaft is rotated with the medium torque at
the medium speed by the motor through the speed reducer. When the
tightening torque reaches the snug torque, the rotation of the
output shaft is interrupted by the switching means through the
motor driver means. Thereafter the output shaft is rotated with the
higher torque at the lower speed by the motor through the speed
reducer. When the tightening torque reaches the final torque, the
rotation of the output shaft is stopped by the switching means
through the motor driver means.
The above and other objects, features and advantages of the present
invention will become more apparent from the following description
when taken in conjunction with the accompanying drawings in which a
preferred embodiment of the present invention is shown by way of
illustrative example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flowchart of a control sequence of a nut runner control
method according to the present invention;
FIG. 2 is a block diagram of a nut runner control apparatus
according to the present invention;
FIG. 3 is a diagram showing a time vs. tightening torque curve and
a time vs. rotational speed curve; and
FIG. 4 is a cross-sectional view of a nut runner controlled by the
method and the apparatus of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENT
A nut runner X shown in FIG. 4 is controlled by a nut runner
control apparatus shown in FIG. 2 in accordance with a nut runner
control method shown in FIG. 1.
As shown in FIG. 2, the control apparatus includes a master
controller A and a subcontroller B coupled thereto for controlling
the nut runner X electrically connected to the subcontroller B to
tighten a fastener such as a bolt, a nut, or the like.
The master controller A is set to a slow-start rotational speed
N.sub.S, a higher, predetermined rotational speed N.sub.H, a medium
rotational speed N.sub.M, and a lower rotational speed N.sub.L for
controlling the rotational speed of a motor 1 of the nut runner X,
reverse rotational speeds N.sub.BH, N.sub.BM, N.sub.BL
corresponding to the rotational speeds N.sub.H, N.sub.M, N.sub.L,
respectively, for braking or stopping the rotation of the motor 1,
a switching torque T.sub.C, a snug torque T.sub.S, and a final
torque T.sub.F for selecting the rotational speeds or shutting off
the motor 1, and an upper limit tightening torque T.sub.AU and a
lower limit tightening torque T.sub.AL for determining whether the
final torque T.sub.F is appropriate or not when the tightening of
the fastener is completed. The master controller A also displays a
tightening torque T.sub.O of an output shaft 4 of the nut runner
X.
The subcontroller B comprises a D/A converter means C for
converting the various preset digital signals from the master
controller A into corresponding analog signals. A detector means D
is connected to a torque sensor 2 of the nut runner X for applying
a signal to and receiving a signal from the torque sensor 2 to
detect the tightening torque T.sub.O. A comparator means E is
provided for comparing the detected tightening torque T.sub.O with
the preset torques T.sub.C, T.sub.S, T.sub.F. A decision means F is
provided for determining whether the final torque T.sub.F is
appropriate or not. A switching means G is responsive to output
signals from the comparator E for sequentially switching circuits
for the preset rotational speeds N.sub.S, N.sub.H, N.sub.M, N.sub.L
and operating a switching clutch 7 of a speed reducer of the nut
runner X. A motor driver means H having a servoamplifier SA
responsive to an output signal from the switching unit G is
provided for driving the motor 1 to rotate the nut runner X
selectively with a lower torque at a higher speed, with a medium
torque at a medium speed, and with a higher torque at a lower
speed.
The comparator means E comprises comparators CS1, CS2, CS3 for
comparing analog voltages converted respectively from digital input
signals representing the preset torques T.sub.C, T.sub.S, T.sub.F
by D/A converters C8, C9, C10 of the D/A converter means C with the
measured tightening torque T.sub.O to determine whether the
measured tightening torque T.sub.O has reached the switching torque
T.sub.C, the snug torque T.sub.S, and the final torque T.sub.F,
respectively, latches RS1 through RS7 for switching the comparators
CS1, CS2, CS3, and delay circuits TM1 through TM7 for delaying
operation times.
The switching means G comprises switching circuits SS1 through SS7
for applying, to the servoamplifier SA, analog voltages converted
respectively from digital input signals representing the preset
rotational speeds N.sub.S, N.sub.H, N.sub.M, N.sub.L, N.sub.BH,
N.sub.BM, N.sub.BL by D/A Converters C1 through C7 of the D/A
converter means C, a switching circuit SS8 responsive to an output
signal from the comparator CS1 for applying an output signal to a
solenoid 6 of the nut runner X to operate the switching clutch of
the nut runner X, and an integrating circuit IS for starting the
nut runner X slowly.
The nut runner X shown in FIG. 4 is driven by the motor 1 which has
a pulse transmitter 5 (FIG. 2). The torque sensor 2 having strain
gages 12 is associated with the output shaft 4 for issuing a signal
indicative of the detected tightening torque T.sub.O to the
detector means D. When the solenoid 6 is energized, it moves the
switching clutch 7 into a higher-speed position against the
resiliency of a spring 11 to rotate the output shaft 4 with a lower
torque at a higher speed through first and third planetary gear
mechanisms 8, 10 of the speed reducer. When the solenoid 6 is
de-energized, it allows the switching clutch 7 to be shifted into a
lower-speed position under the bias of the spring 11 for rotating
the output shaft 4 through the first, second, and third planetary
gear mechanisms 8, 9, 10 of the speed reducer.
Operation of the control apparatus for tightening a fastener with
the nut runner X will be described below with reference to FIGS. 1,
2, and 4.
A start signal is applied from the master controller A to the latch
RS1 to set the same to close the switching circuit SS8 for
energizing the solenoid 6 of the nut runner X. The switching clutch
7 of the nut runner X is shifted into the higher-speed position to
ready the nut runner X for low-torque high-speed operation in a
two-stage speed reduction mode achieved by the first and third
speed planetary gear mechanisms 8, 10.
After the switching clutch 7 has been shifted, the delay circuit TM
closes the switching circuit SS1 to reset the latch RS1 and to set
the latch RS2. When the switching circuit SS1 is closed, the
digital signal representing the preset slow-start rotational speed
N.sub.S is converted into an analog voltage signal by the D/A
converter C1, and the analog voltage is issued via the integrating
circuit IS to the servoamplifier SA, which employs the applied
voltage as a rotation control voltage to control the voltage from a
motor power supply through a feedback control loop for smoothly and
slowly starting to energize the motor 1. Then, upon elapse of a
slow-start time, the delay circuit TM8 opens the switching circuit
SS1 to stop the slow-start rotation of the motor 1. An output
signal from the latch RS2 is applied to the delay circuit TM2 which
closes the switching circuit SS2 at the same time that the
slow-start rotation of the motor 1 is stopped. An analog voltage
converted from the digital signal representing the higher
rotational speed N.sub.H by the D/A converter C2 is applied through
the switching circuit SS2 to the servoamplifier SA, which controls
the voltage applied to the motor 1 to rotate the motor 1 with a
lower torque at a higher speed.
The output shaft 4 which is rotated by the motor with the lower
torque at the higher speed generates the tightening torque T.sub.O
for tightening the fastener. The generated tightening torque
T.sub.O is detected by the torque sensor 2 coupled to the output
shaft 4, and its signal is applied to the detector means D of the
subcontroller B. The detector means D applies a carrier voltage
from an oscillator OS through an amplifier AMP to the strain gages
12 of the torque sensor 2. The strain gages 12 are connected as a
Wheatstone bridge which is kept in equilibrium unless the
tightening torque T.sub.O is produced by the output shaft 4. When
the tightening torque T.sub.O is generated by the output shaft 4,
the Wheatstone bridge is brought out of equilibrium and detects the
strain on the output shaft 4. A signal representing the strain is
applied from the torque sensor 2 to a synchronous detector SY
through an amplifier AMP. The carrier of an output signal from the
synchronous detector SY is cut off by a filter FT. The detector
means D now produces a voltage signal representing the tightening
torque T.sub.O. The detector means D applies the tightening torque
voltage to the comparator means E. The tightening torque voltage
from the detector means D is applied to the comparator CS1 for
comparison with the analog voltage converted from the digital
switching torque T.sub.C.
When the tightening torque T.sub.O reaches the switching torque
T.sub.C, the latch circuit RS2 is reset and the switching circuit
SS2 is opened, and the latch circuit RS5 is set and the switching
circuit SS5 is closed. At this time, an inverted voltage of the
reverse rotational speed N.sub.BH is applied via the switching
circuit SS5 to the servoamplifier SA to forcibly reversing the
motor 1 to quickly brake or stop the rotation of the motor 1. The
period of time for which the switching circuit SS5 is closed, i.e.,
the rotation of the motor 1 is stopped, is preset in the delay
circuit TM3. After elapse of this period of time, the switching
circuit SS5 is opened and the latch circuit RS5 is reset. The
output signal from the comparator CS1 is also applied to open the
switching circuit SS8 to de-energize the solenoid 6 of the nut
runner X, retracting the speed reducer of the switching clutch 7
from the higher-speed position.
Since the switching clutch 7 is urged toward the lower-speed
position by the spring 11, the switching clutch 7 is automatically
shifted into the lower-speed position, and now the output shaft 4
can be rotated with a medium torque at a medium speed or with a
higher torque at a lower speed in a three-stage speed reduction
mode achieved by the first through three planetary gear mechanisms
8, 9, 10.
The output signal from the comparator CS1 is also supplied to set
the latch RS3 to energize the delay circuit TM4. After elapse of
the shutdown time of the motor 1, the delay circuit TM4 closes the
switching circuit SS3 to allow the analog voltage representing the
medium rotational speed N.sub.M from the D/A converter C3 to be
applied via the switching circuit SS3 to the servoamplifier SA. The
servoamplifier SA then controls the voltage applied to the motor 1
to rotate the output shaft 4 with the medium torque at the medium
speed.
When the tightening torque T.sub.O detected by the torque sensor 2
reaches the snug torque T.sub.S, a signal indicative of the
tightening torque T.sub.O is supplied from the detector means D and
issued from the comparator CS2. The output signal from the
comparator CS2 resets the latch RS3 and opens the switching circuit
SS3, and at the same time sets the latch RS6 and closes the
switching circuit SS6. The medium-torque medium-speed rotation of
the motor 1 is now discontinued, and upon elapse of a preset period
of time, the switching circuit SS6 is opened by the delay circuit
TM5 and the latch RS6 is reset.
At this time, the output signal from the comparator CS2 sets the
latch RS4 and is applied to the delay circuit TM6. After the motor
1 is shut off for the preset period of time, the delay circuit TM6
closes the switching circuit SS4 to enable the motor 1 to rotate
the output shaft 4 with the higher torque at the lower speed.
The tightening torque T.sub.O is increased by the higher-torque
lower-speed rotation of the output shaft 4. When the tightening
torque T.sub.O reaches the final torque T.sub.F, a signal
indicating the tightening torque T.sub.O is supplied from the
detector means D and issued from the comparator CS3 to reset the
latch RS4 and opens the switching circuit SS4. Simultaneously, the
latch RS7 is set and the switching circuit SS7 is closed to
discontinue the higher-torque lower-speed rotation of the output
shaft 4.
In order not to reverse the motor 1, the switching circuit SS7 is
opened and the latch RS 7 is reset by the delay circuit TM upon
elapse of a preset period of time.
The maximum level of the tightening torque T.sub.O detected by the
torque sensor 2 is held by a peak hold circuit PS of the decision
means F. The analog voltage indicative of the maximum tightening
torque level is converted by an A/D converter into a digital
voltage which is then displayed as a digital value on a tightening
torque display (not shown) in the master controller A. The peak
hold circuit PS is initially reset by the start signal applied
thereto through the latch RS1. The output signal from the peak hold
circuit PS is also applied to a window comparator WC which compares
the maximum tightening torque level with the upper limit tightening
torque T.sub.AU and the lower limit tightening torque T.sub.AL
which are supplied from the master controller A. If the maximum
tightening torque falls between the upper and lower limit
tightening torques T.sub.AU, T.sub.AL, then the window comparator
WC applies an OK signal to the master controller A. If the maximum
tightening torque falls outside the range between the upper and
lower limit tightening torques T.sub.AU, T.sub.AL, then the window
comparator WC applies an NG signal to the master controller A. The
decision means F thus determines whether the tightening torque is
appropriate or not.
A pulsed output signal from the pulse transmitter 5 associated with
the motor 1 is converted into a voltage by a pulse-to-voltage
converter PC. The voltage is fed back to the servoamplifier SA for
controlling the rotation control voltage from the switching means G
to energize the motor at a selected one of the rotational speeds
and produce a selected one of the torques.
In order to set the final torque T.sub.F to an appropriate torque
level, performing a tightening procedure efficiently, and prevent
excessive tightening of the fastener, the fastener is tightened
with a lower, predetermined torque at a higher, predetermined speed
immediately before the fastener is seated on a surface since the
tightening torque is not large, and when the tightening torque
reaches the preset switching torque T.sub.C, the nut runner output
shaft is temporarily stopped. Then, the fastener is tightened again
with a medium torque at a medium speed so that it is seated with
the medium torque relative to the lower, predetermined torque and
at the medium speed relative to the higher, predetermined speed.
When the tightening torque reaches the preset snug torque T.sub.S,
the nut runner output shaft is stopped again, after which the
fastener is tightened again with a higher torque relative to the
lower, predetermined torque and at a higher speed relative to the
higher, predetermined speed until the tightening torque reaches the
final torque T.sub.F. The tightening time and the tightening
condition are good by selecting the snug torque T.sub.S to be about
20% of the final torque T.sub.F and also selecting the preset
switching torque T.sub.C to be about 70% of the preset snug torque
T.sub.S.
FIG. 3 shows a graph having a horizontal axis representing time and
a vertical axis representing the detected tightening torque. An
upper t-T.sub.O curve indicates how the tightening torque T.sub.O
varies with time, and a lower t-N curve shows the manner in which
the rotational speed N of the output shaft 4 varies with time.
The t-T.sub.O curve is plotted when the snug torque T.sub.S is 20%
of the final torque T.sub.F and the switching torque T.sub.C is 70%
of the snug torque T.sub.S. With the illustrated t-T.sub.O curve,
the fastener such as a bolt is tightened in a high speed range from
600 rpm to 800 rpm, preferably from 700 rpm to 800 rpm, and then
abruptly stopped by applying an inverted voltage to the motor at
the preset switching torque T.sub.C immediately before the fastener
is seated. Therefore, the tightening torque is stable just before
the fastener is seated. Then, the fastener is tightened in a medium
speed range from 100 to 200 rpm, and the tightening torque is
increased up to the preset snug torque T.sub.S which is reached in
the fastener seated condition. The motor 1 energized again to
rotate the output shaft 4 in a low speed range from 10 to 30 rpm to
tighten the fastener, which is abruptly stopped when the preset
final torque T.sub.F is reached. Since no response delay is
experienced and the fastener is tightened from the stopped
position, the tightening torque T.sub.O, i.e., the final torque
T.sub.F, is made substantially constant, and hence the tightening
procedure can easily be supervised. With the medium rotational
speed being set, the higher rotational speed can be increased from
conventional 300 rpm to a speed ranging from 600 rpm to 800 rpm, so
that the total tightening time can be shortened. While the final
torque T.sub.F varies with the material of the fastener to be
tightened, dependent on whether a washer is employed or not, with
the pitch of the screw threads of the fastener, the condition of a
surface on which the fastener is to be seated, and various other
factors, the values to which the master controller A is to be set
can easily be varied by way of digital input signals. By modifying
the preset values, the tightening procedure can easily be
supervised and the fastener can be tightened with high
accuracy.
The control apparatus shown in FIG. 2 is shown as being of a
sequence control arrangement, but may comprise a microprocessor so
that the control process can be programmed by software.
With the present invention, as described above, the tightening
torque of the output shaft 4 is detected, and when the detected
tightening torque reaches preset torque levels, the rotation of the
output shaft 4 is interrupted. The torque and rotational speed of
the output shaft 4 are switched between three torques and three
rotational speeds, i.e., a lower torque and a higher speed, a
medium torque and a medium speed, and a higher torque and a lower
speed. Therefore, a response delay in switching the rotational
speeds and adverse effects of an inertial force of the output shaft
can be eliminated. Since the higher rotational speed may be further
increased, therefore, the time required for tightening the fastener
can be reduced.
Although a certain preferred embodiment has been shown and
described, it should be understood that many changes and
modifications may be made therein without departing from the scope
of the appended claims.
* * * * *